I propose to identify cell surface antigens that are present on chemically transformed mouse fibroblasts but not on normal fibroblasts. My strategy will be to fuse myeloma cells with spleen cells from rats immunized with fibroblasts transformed in vitro with methylcholanthrene, so as to produce antibody-secreting somatic cell hybrids (hybridomas). Hybridomas producing monoclonal antibodies that bind to the transformed fibroblasts used for immunization, but not to normal fibroblasts, will be identified by an autoradiographic indirect 125I-protein A binding assay and cloned. The monoclonal antibodies will be purified either from spent culture medium or from the ascites fluid of nude mice or rats bearing hybridomas. The purified antibodies will be labelled with 125I and used to test a number of transformed and revertant fibroblast lines. Antibodies that identify transformation-specific antigens will be tested on a more extensive panel of cell types. The antigens identified will be purified from radiolabelled cells by immunoadsorption on antibody-Sepharose columns and sodium dodecyl sulfate-polyacrylamide gel electrophoresis for subsequent structural analysis, which will comprise molecular weight determination, peptide mapping, and partial amino acid sequence determination.